“Mode Division Multiplexed Transmission with a weakly-coupled Few-Mode Fiber”, OFC/NFOEC Technical Digest 2012 OTu2C.5, by Massimiliano Salsi, et al. (Non Patent Literature 1) discloses an optical fiber transmission system that is capable of performing weakly-coupled mode division multiplexed transmission using, as a transmission line, a few-mode fiber with reduced crosstalk (hereinafter referred to as “XT”) between spatial modes. Instead of adopting a multi-input-multi-output (MIMO) configuration involving collective input and output of all spatial modes, this system adopts a MIMO configuration for each of four LP modes, LP01, LP11, LP21, and LP02.
Examples of an optical fiber that can be used as a transmission line for mode division multiplexed transmission include not only multi-mode fibers, such as few-mode fibers, but also include multi-core fibers (hereinafter referred to as “MCFs”). For example, “114 Space-Division-Multiplexed Transmission over 9.8-km Weakly-Coupled-6-Mode Uncoupled-19-Core Fibers”, OFC Postdeadline Papers 2015 Th5C.4, by Koji Igarashi, et al. (Non Patent Literature 2) discloses an optical fiber transmission system that is capable of performing weakly-coupled mode division multiplexed transmission using, as a transmission line, an MCF with 19 cores (hereinafter, MCF with “n” cores is referred to as “n-core MCF”) each configured to guide six LP modes (four degenerate LP modes). Instead of adopting a MIMO configuration for each of the LP modes, this system adopts a MIMO configuration for LP01 mode and LP11 mode and a MIMO configuration for LP21 mode and LP02 mode. Spatial modes guided in the 19-core MCF are the following six LP modes: LP01 and LP11 (identifiable by two degenerate modes, even mode LP11a and odd mode LP11b, having the same propagation constant), LP21 (even mode LP21a and odd mode LP21b), and LP02.
Also, “2.05 Peta-bit/s Super-Nyquist-WDM SDM Transmission Using 9.8-km 6-mode 19-core Fiber in Full C band”, ECOC 2015 PDP.3.2 by D. Soma, et al. (Non Patent Literature 3) discloses an optical fiber transmission system that uses, as a transmission line, a 19-core MCF with cores each configured to guide the six LP modes disclosed in Non Patent Literature 2. This system adopts a MIMO configuration which involves collective input and output of all spatial modes. The MIMO configuration involving collective input and output of all spatial modes is adopted here, because adopting a MIMO configuration for each spatial mode group results in insufficient XT compensation.
“Splice loss requirements in multi-mode fiber mode-division-multiplex transmission links”, OPTICS EXPRESS 2013 Vol. 21, No. 1 519, by Stefan Warm and Klaus Petermann (Non Patent Literature 4) discloses a mode division multiplexed transmission system that uses, as a transmission line, an optical fiber which is not limited to a weakly-coupled fiber. It has been pointed out that in view of mode dependent loss (MDL) and inter-mode loss difference during connection of optical fibers forming a transmission line, it is necessary in this system to reduce axial offset during fiber connection.